Experiment 6

Liquid Nitrogen-Assisted, High-Vacuum Pumpdown

 

Objective: Experimentally obtain the pumpdown curve with the assist of a liquid nitrogen cryopanel. Practice safe handling practices and procedures for cryogenic fluids.

Reference: Modules 1, 4, 6, and 10

Required Materials:

Varian Vacuum Trainer

Stopwatch

Latex gloves

Liquid nitrogen

Copper tubing coil

Liquid nitrogen safety gloves and safety glasses

Dewar

 

Experimental Procedure:

Students must wear latex gloves when handling vacuum components.

1. Install the copper tubing coil on the RF feedthrough.

2. Connect a liquid nitrogen source to the RF feedthrough. Do NOT fill with liquid nitrogen at this time.

3. Insure that the system has achieved equilibrium at atmospheric pressure. With all valves closed, remove the bell jar and let stand for at least 15 minutes.

4. Visually inspect the base plate and gasket for cleanliness. If the gasket needs cleaning, wipe with alcohol. Dry thoroughly with a lint-free cloth and lubricate with a thin film of vacuum grease.

5. Place the bell jar and shield on the baseplate.

6. Close all valves.

7. Plug the trainer into the power outlet.

8. Turn both Mechanical and Turbo Pump toggle switches to the ON position.

7. Select the roughing gauge on the controller (TC1, Channel 1).

8. Check the gauge. It should be reading atmospheric pressure (7.6 on the +2 scale). Note: The TC gauge is not accurate above 1 Torr.

9. Open the foreline valve.

10. Press START on the Turbo Controller Panel. Allow the Turbo Controller to come up to speed ( 75 K rpm) and indicate NORMAL OPERATION.

11. Close the foreline valve.

12. Open the roughing valve in one continuous motion. Note: The foreline and roughing valve should never be opened at the same time.

13. At 3 Torr, add liquid nitrogen to the cryopanel.

14. Begin crossover operation by

- closing the roughing valve

- opening the foreline valve and waiting 10 seconds

- opening the high vacuum valve and start the stopwatch

15. Switch the gauge to BA (ION gauge). The gauge will begin operation when in range (<1 mtorr).

16. Continue taking readings until reaching base pressure. Record the pressure at 30 second intervals for 5 minutes, at 1 minute intervals for the next 5 minutes, and 2 minute intervals until completion. (Minimum length of the experiment is 20 minutes.)

17. Plot pressure versus time from crossover on seven-cycle x 70 division semilog graph paper.

18. Compare the data below crossover obtain in this experiment with the data obtained in Experiment 3.